1. Field of the Invention
This invention relates to a canister and an evaporative fuel-processing system employing the same, and more particularly to a canister and an evaporative fuel-processing system, which adsorb evaporative fuel generated in a fuel tank of an internal combustion engine for vehicles, to thereby prevent evaporative fuel from being emitted into the atmosphere.
2. Prior Art
Canisters employed in conventional evaporative fuel-processing systems for internal combustion engines (hereinafter referred to as "the engine") for vehicles include a U-shaped flow-type canister as shown in FIG. 1, which is known, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 1-159455. The canister 1 includes a casing 2 in the form of a rectangular parallelepiped. The casing 2 has a top wall 3 formed integrally with a plurality of protuberances 4 formed by embossing an inner surface of the top wall 3 and extending downward from the inner surface. The casing 2 has a chamber 5 and a chamber 6 formed therein and partitioned from each other by a partition member 7. The chambers 5 and 6 communicate with each other through a space 16 defined below a lower end of the partition member 7. Filters 8, 9 are mounted in the chambers 5, 6, respectively, in a fashion abutting on respective corresponding ones of the protuberances 4. Spaces 18 and 18' are defined in the top wall 3 between the inner surface of the top wall 3 and the filter 8, and between the inner surface of the top wall 3 and the filter 9, respectively.
A punched metal member 12 is mounted in a lower portion of the casing 2 and held in spaced relation to a bottom wall 13 of the casing 2 by a coiled spring 14 which upwardly urges the punched metal member 12, as viewed in the figure, to thereby define the space 16. A filter 10 is mounted on the punched metal member 12 at a lower end of the chamber 5, and a filter 11 on the punched metal member 12 at a lower end of the chamber 6. The chamber 5 is partly defined by the filters 8 and 10, and the chamber 6 by the filters 9 and 11, respectively, and are filled with activated carbon 15 as adsorbents.
A charging port 20, a high-speed charging port 21, and a purging port 22 are formed in the top wall 3 of the casing 2, at locations corresponding to the chamber 5. The charging port 20 is connected to a fuel tank 25 through a charging passage 24, and the high-speed charging port 21 to the fuel tank 25 through a charging passage 26 with a control valve 32 arranged thereacross for opening and closing the same. During refueling, the control valve 32 opens due to increased pressure of evaporative fuel in the fuel tank 25, or the valve 32 is opened by an electronic control unit (ECU), not shown. The purging port 22 is connected to an intake system 28 of the engine through a purging passage 27. An atmospheric air port 23, which opens into the atmosphere, is formed in the top wall 3 of the casing 2, at a location corresponding to the chamber 6.
Arranged across the charging passage 24 is a two-way valve 30 which is comprised of a positive pressure valve which opens when the pressure within the 35 fuel tank 25 is higher than that within the canister 1 by a predetermined amount or more, to allow evaporative fuel within the fuel tank 25 to flow into the canister 1, and a negative pressure valve which opens when the pressure within the fuel tank 25 is lower than that within the canister 1 by a predetermined amount or more, to allow evaporative fuel to flow from the canister 1 into the fuel tank 25.
An electromagnetic valve 31 is arranged across the purging passage 27, which is controlled by the ECU to control the flow rate of evaporative fuel purged through the purging port 22 and the purging passage 27 into the intake system 28 of the engine, according to operating conditions of the engine.
During parking of a vehicle in which the engine is installed, with the engine in stoppage, or during operation of the engine, evaporative fuel generated in the fuel tank 25 is introduced through the charging passage 24 and the charging port 20 into the chamber 5 of the canister 1. Most of the evaporative fuel is adsorbed by the activated carbon 15 accommodated within the chamber 5, and then the remaining part of the evaporative fuel overflows from the chamber 5 and is introduced through the space 16 in the lower portion of the casing 2 into the chamber 6 to be adsorbed by the activated carbon 15 within the chamber 6.
During refueling, a large amount of air containing evaporative fuel is introduced from the fuel tank 25 through the charging passage 26 and the high-speed charging port 21 into the chamber 5 of the canister 1. The large amount of air introduced into the chamber 5 then flows through the space 16 into the chamber 6, wherefrom it is discharged through the atmospheric air port 23 into the atmosphere. Evaporative fuel contained in the air is adsorbed by the activated carbon 15 accommodated within the chambers 5 and 6 while the air flows through the chambers 5 and 6.
In the conventional U-shaped flow-type canister constructed as above, however, the flow rate of air containing evaporative fuel flowing into the canister 1 through the high-speed purging port 21 during refueling is about 1000 times as large as the flow rate of evaporative fuel flowing into the canister 1 through the charging port 20 during operation of the engine. Therefore, during refueling, the flow velocity of air containing evaporative fuel passing through the adsorbent in the canister 1 is high. As a result, the adsorbing efficiency of the canister 1 is much degraded. The rate of degradation is as large as approximately 50%.